Abstract: The present invention provides CD20-binding proteins that bind to and rapidly internalize in a CD20-mediated fashion from a cell surface location to the interior of the cell. CD20-binding proteins of the invention comprise a CD20 binding region and a Shiga toxin effector region. Certain of the disclosed CD20-binding proteins kill cells that express CD20 on their surface. Further, the presently disclosed CD20-binding proteins can comprise additional exogenous materials, such as, e.g., antigens, and are capable of targeted delivery of these additional exogenous materials into the interior of CD20 expressing cells. These CD20-binding proteins have uses in methods such as, e.g.